DE 10 2006 006 185 A1 describes a packaging machine, which comprises transport means transporting a film web by a certain feed length in a cyclic manner in a direction of transport and in which said feed length can be modified according to the lengthening of the transport means. Such packaging machines may comprise e.g. forming, filling, sealing and separating stations. Since such machines often have a length of from 10 m to 20 m and more, the lengthening of the transport means, which are normally configured as transport chains, becomes increasingly important as regards precise guidance of the film web through the packaging machine.
When the two chains, which fix and guide the packaging film on its outer edges, lengthen, the packaging trays formed in the film will be displaced in the direction of transport. This displacement may interfere with the correct sequence of packaging steps to such an extent that the packaging in question will no longer fulfil the respective quality standards and will have to be rejected.
In order to avoid such failures, this publication therefore suggests that the lengthening of the transport means should be measured and that its feed length should be adapted on the basis of this measurement.
The same topic underlies DE 101 52 891 B4, which suggests that, in response to a change in the chain length, the feeding step of the transport unit should be changed or the distance between a forming station and a punching station should be varied. The tensile stress of the transport chain is maintained constant throughout the whole conveying step. The relative position of a chain tensioner tensioning the transport chain is determined by means of a sensor and transmitted to a control unit, so that the latter will be able to detect an e.g. temperature-dependent variation of length of the chain and/or a variation of the tensile force of the transport chain. It follows that this detection of the chain condition depends on two parameters. On the one hand, the change in the chain length and, on the other hand, the pretensioning force applied to the chain tensioner. The control unit is, however, not able to detect in this way which of the parameters changed.
DE 10 2005 044 537 B3 discusses a change in the length of the transport chains for thermoforming machines. In this publication it is suggested that, during the transport step of the film, a reduced tensioning force should be applied to the transport chains so as to reduce the frictional resistance and the wear of the chain. At standstill, an increase in the tensioning force is to be effected so that an identical reproducible force will always prevail in the two transport chains and so that comparable values and, consequently, a change in the length of the transport chains can be measured via a position measuring system.